How a monk must always be eager to preserve his purity of heart.
It is like the case when one endeavours to strike some mighty prize of virtue on high, pointed out by some very small mark; with the keenest eyesight he points the aim of his dart, knowing that large rewards of glory and prizes depend on his hitting it; and he turns away his gaze from every other consideration, and must direct it there, where he sees that the reward and prize is placed, because he would be sure to lose the prize of his skill and the reward of his prowess if the keenness of his gaze should be diverted ever so little.
Source: Institutes (New Advent)